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colouring matters, arc xubstanth'e. colours, i.e., possess the pro-
perty of attaching themselves to the cotton fibre without the aid
of a mordant. Cf/wt m/.s- and hetizoflitrfiuruis are combina-
tions of bcn/idinc and its homologues with the sulphonic acids
of naphthol and naphthylamine. The following- js the constitu-
tion of Congo red, the simplest of these compounds, which is
used in the form of its sodium salt : 

N:N. (',!!,



Attention should be drawn to the fact that azobenzenc,
although a, brightly coloured substance, is without dyeing' pro-
perties, />., it is not a colouring matter, whereas aminoazobenz-
ene and methyl onmgc are true dyes. They all three contain
the a/o-group ( N:N \ called by Witt a chroniophore, united
to two aromatic nuclei ; but in the case of aminoaxobenzene
and metli\'l oi'an^e, (Hie of these nuclei contains a basic group,
NIL, or N(('II:,),,. Ii will also have been observed that the
combinations with phenols likewise result in the production of
colouring matters. It. would appear, therefore, as if there were
at least two essentials to a dye, a fundamental or mother sub-
stance like axobenxene, termed a chroniogenic compound, and
an ammo- or hydroxyl group, called an aitxochromc. The same
thing has been observed in the case of other colouring matters
(see Note on I 'rep. 103, p. 313).
Most of the a/.o-colours split at the double link, on reduction
with stannous chloride and hydrochloric acid, forming two
molecules of base. Methyl orange yields sulphanilic acid and
dimethyl /-phcnylenediamine,
S(;.,li.CViriN:NA%,;lI4N(CU.i).. I all.,
' = SO..I I.C0U4Nlla +